1. Field of the Invention
The invention relates generally to pavement or slab construction and, more specifically, to a concrete joint restraint system for securely supporting dowels of any material between concrete reinforcing bar "bolsters" (used primarily to support spaced layers of rebar on top of one another) and are also known to the construction art as concrete reinforcing bar supports, or "high chairs", the concrete joint restraint system being for use in jointed pavement construction as well as various other jointed concrete applications.
2. Description of Related Art
The present invention pertains to improvements in the field of pavement construction such as those designed for highway transportation. It is well known that concrete has a comparatively high compressive strength, a comparatively low tensile and shear strength, and that concrete expands and contracts due to changes in temperature. Because highways can experience large temperature changes over the course of a calendar year, accommodations must be made for the resulting changes in the concrete. For example, winter temperatures may cause the concrete to experience subzero temperatures, while the same concrete may be exposed to temperatures of over 100.degree. F. in the summer. The thermal expansion and contraction of the concrete under these conditions can prove destructive, leading to cracking and surface discontinuities if the proper precautions and measures are not taken. In addition concrete will crack naturally, as a result of the curing process, which takes place from the time of placement of the fluid concrete material, until full design strength is achieved. This occurs usually within one month after initial placement of the concrete.
Engineers have found that a series of concrete blocks or slabs positioned with a gap to relieve the stresses in the blocks at the maximum expansion anticipated provides the best solution to these problems. Joints or spacing in between the blocks are necessary to accommodate thermal expansion and contraction of the concrete due to changes in the environmental temperatures, and strategic placement of the joints assist engineers in controlling the direction of the expansion and predicting the location where the concrete will crack as a result of the curing process.
The use of discrete blocks, however, is not without its own problems. Uneven expansion or contraction of the individual blocks can result in discontinuities in the highway which, in turn, can lead to unsatisfactory road conditions as well as stress and fatigue in the individual blocks. Blocks can shift up to create unsafe road conditions and reduce the life of the road. To solve this problem, the construction of concrete pavements have for a long time used dowel bar inserts as load transfer mechanisms. As early as 1917, dowel bars have been used to transfer shear loads at joints in the concrete blocks which make up the pavement. Dowels placed longitudinally in the blocks allow the concrete blocks, or slabs, to expand in the longitudinal direction but resist expansion in the traverse direction of the dowels. By controlling the direction of the expansion of the slabs, engineers can prevent the driving surface of the pavement from becoming discontinuous and uneven.
Previously, steel dowels were the exclusive material used for the joint restraint of the concrete. Steel dowels are relatively cost effective and provide the necessary strength required by this application. However, steel dowels have a corrosive tendency when exposed to the harsh environments of the highway, such as salt, oil, dirt, and moisture which seeps between the joints and attacks the dowels. Corrosion results in the dowel binding because the concrete can no longer expand along the dowels, which severely reduces the load transfer efficiency and can also result in the failure of the dowel if the stresses become large enough.
Various dowel protective coatings have been used to prevent corrosion at the dowel-concrete interface. In addition to preventing corrosion, coatings promote movement in the longitudinal direction which increases the load transfer efficiency. The ideal coating would have a low coefficient of friction with the concrete and a high resistance to corrosion, be safe to work with, and be economical. Both powders and epoxy resins have been used with some success in the art, but no ideal coating has been found to date. The biggest problem is that the most effective coatings are often times harmful to the environment or fail to meet strict code requirements.
The industry has used so-called insertion machines that literally press dowels into wet concrete along the intended joint line. Steel bars, however, have a relatively heavy unit weight (e.g. about 600 lbs. per cubic foot) such that the dowels tends to sink into the concrete if placed there with an insertion machine.
Consequently, the industry often uses so-called "dowel baskets" comprised of cane legs, runners and welded dowels to position the dowel bars at the desired height prior to placement of the concrete, for either a slab/pavement expansion or contraction situation, as determined by design. For pavement construction, slab/pavement subgrade is accurately graded and a basket or cage is placed on the subgrade. The dowel bars are in parallel alignment and form a part of the dowel basket and are thus affirmatively held at the desired height and location when the concrete is placed in position. The dowels are preferably positioned so that the midpoint of the dowel lies at the joint or juncture of two adjacent slabs of concrete at the midpoint of the slab vertical dimension. In this manner the slabs are permitted to move horizontally in the longitudinal direction of the dowels into the gap provided for at the joint, but vertical or lateral movement of the slabs is restrained. Whatever method is used to insert the steel dowels, the problem of corrosion remains.
Fiberglass dowel bars and Fiber Composite (FC) dowels have been recently tested in laboratories to replace the steel dowel bars. Fiberglass dowels are much less susceptible to corrosion than the steel counterparts and, thus, they do not require coatings which can be harmful to the environment. The current cost of fiberglass dowel bars can exceed the cost of steel dowels. Aside from the cost considerations, Fiberglass Reinforced Polymer (FRP) dowels have been shown to compare favorably with steel bars in terms of performance.
FRP dowels are desirable because they eliminate most corrosion issues. There have been a number of attempts to secure FRP dowels to steel baskets using spring clips, plastic tie devices. All of the known methods, however, have drawbacks. For example, they often do not grip the FRP material securely; the dowel assemblies cannot be assembled in a factory and shipped to the jobsite without unacceptable transit damage; and they are likely to be damaged during installation or during concrete placement. The FRP dowels assembled into dowel assemblies using the prior methods tend be become loose and misaligned.
There remains a need, therefore, for a concrete joint restraint system or dowel bar assembly which can effectively secure dowels of any material to bolsters made of any material including, specifically, the ability of securing FRP dowels to steel wire bolsters.